Differential circuits are used in a variety of circuit applications to provide common mode rejection of noise produced by power supplies. In addition, differential circuits have twice the dynamic range of single ended circuits. Because of these advantages, differential circuits are used in a variety of noise sensitive applications, such as voltage control oscillators, filters, analog-to-digital converters, band-gap references, or voltage reference circuits.
While differential circuits offer the above-mentioned advantages, there are certain problems with using such circuits. For example, the differential circuits may produce a common mode error due to balancing errors with the differential circuit, noise, or input variations. The common mode error causes the outputs of the differential circuits to drift and, depending on the amount of drift, the outputs may distorted beyond usability. This is particularly disadvantageous for differential circuits that are used in voltage control oscillators.
In a voltage control oscillator, the differential circuit provides an output frequency reference based on a received input voltage signal. The input voltage signal is converted into a current which is used to charge and discharge a capacitor. The voltage impressed upon the capacitor provides the input signals to the differential circuit, wherein the direction of the current, and thus the charging and discharging, is controlled by a differential threshold detector that switches the current direction when one of the differential output reaches a certain threshold. Thus, the voltage controlled oscillator provides a regulated frequency signal based on the received input voltage signal.
When common mode error is present within the differential circuit, the average of the differential outputs is not at a desired level. Under this condition, the voltage controlled oscillator is slowed and is not operating at an optimal level. This is further complicated as the supply voltage is reduced, which is a popular design requirement in an effort to minimize power consumption.
As the common mode error increases, the above mentioned effects likewise increase until the voltage controlled oscillator is inoperable. This occurs when the common mode, or average signal value, of the differential output approaches one of the voltage supply rails. As the differential output approaches a supply rail, one output is practically lost causing the differential threshold detector to detect only one differential output and not the other. Thus, shutting down the voltage controlled oscillator.
One approach for to limit common mode error is to couple a common mode feedback amplifier to the differential circuit. The common mode feedback amplifier minimizes common mode error by sensing, via a switched capacitor sensing circuit, the common mode of the differential output and correcting any errors sensed. While this approach works well in many applications, it does not work well when the output frequency of the differential circuit is high because the switching capacitors must be switched at a rate much higher than the frequency of the signal that is being sensed. In a high frequency application, where the signal to be sampled is the greatest frequency in the circuit, the switching capacitor sensing circuit will not work.
Therefore, a need exists for method and apparatus that will sense and correct common mode errors of differential circuits used in high frequency applications without slowing the differential circuits operation as the supply voltage is reduced.